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A few years ago a zany scientist called Roberto Volterri published a book claiming we all need to look at Italian Old Master paintings more carefully. Why? Because in many of them, celestial bodies long passed off as clouds or birds were actually UFOs. The heavenly dome in Francesco Botticini's Assumption of the Virgin, for instance, was a flying saucer.

Botticini's vast altarpiece – once in Florence’s San Pier Maggiore church and these days in the National Gallery collection – is now the subject of an exhibition. It plainly does not feature alien life, but there's still a fascinating story to be told.

The painting was commissioned by Matteo Palmieri (1406-1475), a top Florentine politician and ally of Cosimo de’ Medici. Over 44 years, he served in all manner of governmental roles, most prestigiously Gonfaloniere di Giustizia (Standard-bearer of Justice).

Palmieri was your quintessential Renaissance Man, also running an apothecary business and writing poetry, histories and philosophical treatises. He commissioned The Assumption just before he died, for his funerary chapel at San Pier Maggiore - intending it, at two metres high by four metres long, as a way of ensuring a literally sizeable legacy.

Its lower register is set in a panoramic landscape just north-east of Florence. The 12 Apostles stand amazed before Mary's opened tomb: it contains not her body but clusters of lilies. The Virgin herself can be seen in the painting's upper register, surrounded by angels and saints as she's crowned Queen of Heaven.

The Virgin adoring the Christ Child, Francesco Botticini, 1470-75

The Assumption stands out for many reasons. For a start, it offers one of the first topographically accurate views in Italian painting. For anyone familiar with Florence, it’s gorgeous fun picking out features that still exist today: the Cathedral, River Arno, Palazzo Vecchio, Palazzo del Bargello, the neighbouring hill town of Fiesole…

It's the painting's upper sphere, however, that is most remarkable. The dome of heaven is illuminated by ethereal golden light. As you approach the painting from afar, it looks less like a canvas on a wall than a painted cupola – in the manner of Brunelleschi's recently completed dome in Florence Cathedral. (Traditionally, artists had represented heaven’s elect in neat, straight rows.)

Of Botticini himself, relatively little is known. Other paintings by him are included, but on evidence such as his two versions of The Virgin Adoring the Christ Child, he never bettered The Assumption. Elsewhere he seems too in thrall to Botticelli, particularly in his fondness for elongated figures and bright, primary colours.

Palmieri died in 1475 and was celebrated city-wide in a grand, state funeral. Botticini completed the altarpiece two years later, including his patron and patron’s wife in the scene, praying on either side of Mary's tomb. The Assumption soon occupied pride of place at San Pier Maggiore.

A few decades later, however, a Church rereading of Palmieri's poem, City of Life, left him suddenly (and posthumously) branded a heretic. Following the early Father, Origen, he’d mooted that human beings weren't conceived by parents but had pre-existed as angels – who were sent to earth to start a new life.

There's nothing explicit in The Assumption that refers to this theology, but its mere association with Palmieri was enough. The chapel had to be closed and the painting covered up, after the latter was vandalised. A photograph of it before a 20th-century restoration reveals that the depiction of Matteo was all but scratched out. A mob reportedly also disinterred his bones from his tomb.

The fervour eventually passed and the altarpiece remained in situ until 1784, when the church was demolished. It was bought by the National Gallery in the 1880s - though hasn’t been on display for many years.

A final section of the exhibition compares Botticini's work with that of his late 15th-century Florentine peers. But this feels rather a dry exercise in curatorial indulgence.

My advice is simply to savour The Assumption. There's more going on in this one painting than the rest of the exhibition combined. It wows us with a little-known tale that reaches peaks of intrigue and artistic achievement utterly typical of Renaissance Italy.

Palmieri's legacy has been secured, though surely not in the manner he expected.